Active noise cancellation (ANC) is a method of reducing an amount of undesired noise received by a user listening to audio through headphones. The noise reduction is typically achieved by playing an anti-noise signal through the headphone's speakers. The anti-noise signal is an approximation of the negative of the undesired noise signal that would be in the ear cavity in the absence of ANC. The undesired noise signal is then neutralized when combined with the anti-noise signal.
In a general noise-cancellation process, one or more microphones monitor ambient noise or residual noise in the ear cups of headphones in real-time, then the speaker plays the anti-noise signal generated from the ambient or residual noise. The anti-noise signal may be generated differently depending on factors such as physical shape and size of the headphone, frequency response of the speaker and microphone transducers, latency of the speaker transducer at various frequencies, sensitivity of the microphones, and placement of the speaker and microphone transducers, for example.
In feedforward ANC, the microphone senses ambient noise but does not appreciably sense audio played by the speaker. In other words, the feedforward microphone does not monitor the signal directly from the speaker. In feedback ANC, the microphone is placed in a position to sense the total audio signal present in the ear cavity. So, the microphone senses the sum of both the ambient noise as well as the audio played back by the speaker. A combined feedforward and feedback ANC system uses both feedforward and feedback microphones.
Typical ANC headphones are powered systems that require a battery or another power source to operate. A commonly encountered problem with powered headphones is that they continue to drain the battery if the user removed the headphones without turning them off.
While some headphones detect whether a user is wearing the headphones, these conventional designs rely on mechanical sensors, such as a contact sensor or magnets, to determine whether the headphones are being worn by the user. Those sensors would not otherwise be part of the headphone. Instead, they are an additional component, perhaps increasing the cost or complexity of the headphone.
The disclosed examples address these and other issues.